Ct (cycle threshold) value indicates the number of cycles completed to identify the virus in a sample during an RT-PCR test. This value is inversely proportional to the viral load in the sample – lower the value, higher the viral presence.
Last week, the state wrote to the Indian Council of Medical Research (ICMR) asking if the Ct value cut-off can be at 24 (which meant a sample testing over 24 can be described as negative). The ICMR responded by saying it cannot be lower than 35.
Dr Balram Bhargava, director general of ICMR, in a letter to state health secretary Dr Pradeep Vyas, said a Ct value of less than 35 should be considered as a Covid-19 positive case, based on a consensus derived from different laboratories in the country. Dr Bhargava wrote that implementing a Ct value cut-off of 24 will lead to “missing of several infectious patients and increased disease transmission.”
Health minister Rajesh Tope told TOI that it has now been decided that those with a Ct value over 35 will be declared Covid-negative. He, however, did not respond to a query on why Maharashtra had sought a clarification on the Ct value in the first place.
Dr Shashank Joshi, a member of the state Covid-19 task force, said there should not be an over-reliance on the Ct value while treating patients. It’s only useful as a diagnostic cut-off and has been reassessed over the past one year, he said. Originally, the ICMR had fixed the threshold at 40 and now it has been revised to 35. “It does not have any relevance for public health or clinical use,” Joshi said.
He said, “The rapid antigen tests come back negative for CT value between 24 and 35. We need to do more and more RT-PCR tests. When there is a large population of asymptomatic patients involved, the question is whether they can be spreaders or not. There is a possibility they can be. Lowering the threshold will reduce the so-called ‘case-demic’. At the same time, those who need care should be quarantined and treated. The idea is to get the right population. “
Dr Suhas Pingle, former IMA (Maharashtra) secretary, said other factors, besides just the Ct value, should be considered while treating Covid patients.
“As clinicians, we consider a patient’s premorbid condition, age, symptoms and severity of infection. The RT-PCR is a diagnostic tool. From the public health point of view, the rapid antigen is a good screening test, but it also throws up a high number of false negatives. The RT-PCR tests are done when someone comes in with a cold, cough, fever or related symptoms, ”he said.